The rice crop is very important as a source of food particularly in highly populated tropical and sub-tropical areas. Its rank in those areas as the principal staple food is as great as that of wheat in northern and temperate areas. The proportion of rice grown in the United States to the total rice crop is small but is increasing especially in Texas, Mississippi, Louisiana, Alabama and Tennessee.
The effective cultivation of rice has been remarkably enhanced by the discovery, as disclosed in U.S. Pat. No. 3,816,092 of a herbicide or weedicide which is very effective in killing both "monocots" and "dicots" and is therefore an excellent herbicide for many crops, but is also extremely selective and specific insofar as non-phytotoxicity to the rice plant and high phytotoxicity to the various weeds other grasses are concerned.
The herbicide or weedicide disclosed in the aforesaid U.S. patent is chemically 3,4-dichloropropioanilide and can also be named N-(3,4-dichlorophenyl) propionamide. However, it is commonly called by the simple name "propanil" and will be so designated hereinafter.
However, the rice crop in many areas which are not and humid, including southern USA and Mexico, is increasingly endangered, not only by weeds, but also by "rice blast" fungus disease (caused by the fungus Piricularia oryzae) which develops two weeks or so after the rice germinates at the very same time as weeds must be controlled by treatment of the crop with propanil. This poses a very serious problem for the agronomist because of the problems encountered when one attempts to treat the new crops simultaneously with propanil and with a fungicide which is effective against rice blast. Many fungicides for other fungus diseases are not effective against Piricularia oryzae and those that are known to have an effect can not be applied to the rice crop in combination with or at the same time as propanil. There are two distinct reasons for this. The first is the fact that most of the chemicals known to be fungicidal to rice blast are carbamates or organo phosphates combined with the further fact that all these fungicides and similar chemicals inhibit the synthesis of the enzyme, aryl acetilamidase which breaks down propanil to 3,4-dichloro propioanilide which is the mechanism that makes propanil harmless to the rice plant even though killing other plants.
The second principal reason why fungicides known to be effective to control rice blast can not be used in combination with propanil resides in the lack of formulation compatibility as between the materials. It has been found that propanil for weed control in rice is best formulated with aqueous sprays based on an emulsifiable concentrate available commercially from the propanil supplier. If the fungicide desired to be used in combination with propanil so as to control both weeds and rice blast is also preparable in form of, or available as, an emulsifiable concentrate, it is obvious that the desired combined treatment by spraying presents no problems. But this is not otherwise the case. For example, the two commercial fungicides known to be most effective in treating rice blast are the material called "Blasticidin" which is basticidin-S benzylamino benzene sulfonate and the material called "Thiabendazole" which is 2-(4'thiazolyl) benzimidazole, and neither of these is available as or so far as is known, can be formulated into an aqueous spray using an emulsifiable concentrate of this material. Instead, their application is from a formulation based on a wettable powder not suitable to obtain an emulsifiable concentrate.